1. Field of the Invention
This invention relates to the preparation of alkaline peroxide solutions and electrolytic cells for the production thereof. In particular, it relates to the manufacture of peroxide bleach solutions having an alkaline concentration such that the solution is suitably directly usable in wood pulp bleaching operations.
2. Description of the Prior Art
Hydrogen peroxide is a strong chemical oxidizing agent whose greatest single use is in the bleaching of cotton and wood pulp.
The production of hydrogen peroxide by the electro-reduction of oxygen has been known since the nineteenth century and the literature contains a vast amount of material on this subject. One of the methods used was that described in Berl, U.S. Pat. No. 2,000,815. Berl carried out the electro-reduction of oxygen on a specially prepared porous plate of active carbon. Oxygen was introduced from one side of the plate and catholyte from the other. The reaction took place on the surfaces of the plate facing the counter electrodes. Strong solutions of potassium hydroxide were used as the catholyte and a porous diaphragm was used to separate the anode and cathode chambers. With a catholyte containing of the order of 20% potassium hydroxide Berl produced 12 to 15% solutions of hydrogen peroxide at a superficial current density of from 0.2 to 0.35 amp./cm..sup.2. However, it was found that when sodium hydroxide was used as the catholyte the results were poor and the special electrode tended to disintegrate.
A more recent procedure of particular interest is that described in Grangaard, U.S. Pat. Nos. 3.454,477; 3,507,769; 3,459,652 and 3,592,749. Grangaard used as an electrode a porous carbon plate with the electrolyte and oxygen delivered from opposite sides for reaction on the plate. His porous gas diffusion electrode requires careful balancing of oxygen and electrolyte pressure to keep the reaction zone evenly on the surface of the porous plate. Moreover, as stated in U.S. Pat. No. 3,507,769, the Grangaard cell gives a peroxide concentration of only 0.5% with an NaOH/H.sub.2 O.sub.2 ratio of 4/1. As described in U.S. Pat. No. 3,459,652, the Grangaard cathode consists of a specially prepared active carbon which is expensive to produce and also deteriorates with time.
Another feature of the Grangaard cell is that it contains an anode and a cathode chamber separated by a semi-previous diaphragm and requires the flow of electrolyte from the anode to the cathode chamber under a small hydrostatic head, to prevent the reaction of peroxide on the anode and a double pass electrolyte feed arrangement as described in U.S. Pat. No. 3,592,749. This has several disadvantages:
1. It complicates the construction of the cell;
2. It increases the electrical resistance of the cell by the resistance of the liquid in the anode chamber;
3. It complicates the operation of the cell, insofar as the flows of both oxygen gas and electrolyte must be continuously balanced for the proper condition to prevail in the cathode chamber. This becomes particularly difficult with flow arrangement as illustrated in U.S. Pat. No. 3,592,749;
4. The oxygen generated at the anode must be collected and pumped back to the cathode.
It is the object of the present invention to provide a simple and inexpensive system for producing alkaline peroxide solutions which will contain about the same amount of alkali as would ordinarily be added to a bleach liquor to adjust the pH of a wood pulp bleaching reaction.